In a radio communication system for communication between mobile objects, such as mobile phones, studies have been done to improve a data rate using various multiplexing techniques. In a recent radio communication system for mobile communication, for example, a frequency domain multiplexing scheme, such as OFDM (Orthogonal Frequency Division Multiplexing) or SC-FDMA (Single-Carrier Frequency Division Multiple Access), which is used for a wireless LAN or digital terrestrial broadcasting. With the use of a frequency domain multiplexing scheme, it becomes possible to suppress deterioration of transmission quality due to fading and to achieve high-speed and high-quality radio transmission. In the frequency domain multiplexing scheme, such as OFDM or SC-FDMA, frequency hopping (FH) may be adopted so as to improve fading resistance. Frequency hopping is a technique for changing a frequency domain to be used from among plural frequency domains at a time interval, making it possible to prevent the use of only a specific frequency domain and to suppress performance deterioration due to frequency-selective fading.
In a cellular system for mobile communication, in an uplink (UL) where upstream communication is performed from a user terminal to a base station, in order to obtain the frequency-diversity effect, a frequency hopping method has been studied in which data is arranged in different frequency resources between the first half slot and the second half slot of the same sub-frame (for example, see NPL 1). FIG. 17 is a diagram showing an operation example of frequency hopping in an uplink. In FIG. 17, (A) shows the allocation of frequency resources, and (B) shows a channel gain |h|2 at that time.
In this example, FIG. 17(A) shows a case where the allocation unit of frequency resource is a resource block (RB), and PUSCH1 which is PUSCH (Physical Uplink Shared Channel) is allocated to a user 1 corresponding to a first user terminal and PUSCH2 is allocated to a user 2 corresponding to a second user terminal. In the drawing, null represents an empty resource in which no data is allocated. Frequency hopping is performed such that, for the channels PUSCH1 and PUSCH2 of the users, resource blocks having different frequencies are respectively allocated to the slots. In this case, from the viewpoint of the channel gain, as shown in FIG. 17(B), different resource blocks are allocated between the first half slot and the second half slot for each user, such that the SINR (Signal-to-Interference plus Noise power Ratio) differs between the slots. Therefore, signals are transmitted using both a low-SINR portion and a high-SINR portion, such that it is possible to average the SINR and to obtain the frequency diversity effect compared to a case where signals are transmitted using only a specific frequency resource.
Although there is an advantage from the viewpoint of performance because of averaging of the SINR through frequency hopping, there are the following problems from the viewpoint of control. First, there is a problem in that the complexity of a scheduler increases so as to search for a pair of user terminals, like the user 1 and the user 2 for hopping in the example of FIG. 17. There is also a problem in that, if the scheduler has not found a pair for hopping, an empty frequency resource (in FIG. 17, a place indicated by null) occurs, causing inefficient use of resources.
With regard to the above-described problems, as a simply resolution of the related art, PVS (Pre-coding vector switching) is applicable. Pre-coding is a transmission beam technique in which, in the case of MIMO (Multiple Input Multiple Output), at the time of transmission from plural antennas, weighted data is transmitted from each antenna to form a beam. PVS is a technique in which the weight (Pre-coding weight) of each antenna at the time of pre-coding is changed and a pre-coding vector is switched. PVS has been studied in a downlink (DL) where downstream communication is performed from the base station to the user terminal (for example, see NPL 2). It is assumed that PVS is applied to the first half slot and the second half slot in the uplink.
FIG. 18 is a diagram showing an operation example of space hopping to which PVS is applied in an uplink. In FIG. 18, (A) shows the allocation of frequency resources and allocation of a pre-coding weight in each frequency resource, and (B) shows a channel gain |h|2 at that time. In this case, the same resource block may be used without changing the frequency resource allocated to each user. In the example of FIG. 18(A), for the first user terminal PUSCH1, different pre-coding weights, such as weights W0 and W1, are applied between the slots. Thus, switching of frequency resource allocation is eliminated, making it possible to resolve the problems in the above-described frequency hopping. In this case, from the viewpoint of the channel gain, as shown in FIG. 18(B), the SINR differs between the first half slot and the second half slot for each user, obtaining the space diversity effect. However, focusing on performance again, there is a problem in that, like the first half slot shown in FIG. 18(B), the SINR of a slot allocated with an inappropriate pre-coding weight significantly decreases, making it difficult to perform demodulation.